1. Technical Field
The present invention relates to an oil or fat containing a triglyceride in which medium-chain fatty acids are bound to positions 1 and 3 of the triglyceride and at least one type of polyunsaturated fatty acid, selected from the group consisting of ω6 series polyunsaturated fatty acids having 18 or more carbon atoms and 3 or more double bonds and ω9 series polyunsaturated fatty acids having 18 or more carbon atoms and 2 or more double bonds, is bound to the 2 position, a production method thereof, and a composition containing these oils or fats.
2. Background Art
Eicosapentaenoic acid (to be referred to as “EPA”) and docosahexaenoic acid (to be referred to as “DHA”) are known to be ω3 series polyunsaturated fatty acids that have numerous physiological functions such as preventive effects on adult diseases such as arteriosclerosis and thrombosis, an anticancer action and an action that enhances learning acquisition, and they have been used in pharmaceuticals and foods for specified health uses. However, there has recently been a growing interest in the physiological functions of polyunsaturated fatty acids other than ω3 series polyunsaturated fatty acids (such as ω6 series and ω9 series polyunsaturated fatty acids).
The pathway by which polyunsaturated fatty acids are biosynthesized in humans consists of two representative series, namely the ω3 series and ω6 series (ω refers to the number of the carbon atom where the first double bond is located counting from the methyl terminal end of the fatty acid). Known examples of ω6 series polyunsaturated fatty acids include linoleic acid, γ-linolenic acid, dihomo-γ-linolenic acid and arachidonic acid.
Arachidonic acid accounts for about 10% of the fatty acids that compose important organs such as the blood and liver (for example, arachidonic acid accounts for 11%, eicosapentaenoic acid 1% and docosahexaenoic acid 3% of the fatty acid composition in the phospholipids of human blood), is involved in regulation of membrane fluidity as a major constituent of the cell membrane, and demonstrates various functions involved in the body's metabolism. On the other hand, it also plays an important role as a direct precursor of prostaglandins. Recently, attention has been focused in particular on the role of arachidonic acid as an infant nutrient by serving as a constituent fatty acid of endogenous cannabinoids that exhibit neurergic action (such as 2-arachidonoyl monoglycerol and anandamide). Although humans are unable to synthesize linoleic acid, following ingestion of vegetable oils, unsaturation and lengthening of the carbon chain are repeated resulting in conversion to γ-linolenic acid, dihomo-γ-linolenic acid and arachidonic acid. Thus, an adequate amount of arachidonic acid is normally synthesized if a diet rich in linoleic acid is consumed. However, in patients with adult diseases, persons susceptible to adult diseases, infants and the elderly, as the activity of enzymes involved in biosynthesis decreases, thus causing a shortage of arachidonic acid, it is preferable to ingest arachidonic acid directly in the form of a composite fatty acid of oils and fats (triglycerides).
Although ingested oils and fats (triglycerides) are typically hydrolyzed by pancreatic lipase when they enter the small intestine, this pancreatic lipase is specific for the 1,3 positions, enabling the 1,3-positions of the triglycerides to be severed resulting in the formation of two molecules of free fatty acid, while at the same time forming one molecule of 2-monoacylglycerol (to be referred to as “2-MG”). Since this 2-MG is extremely soluble in bile acids and has a high degree of absorption, 2-position fatty acids are typically considered to be easily absorbed. In addition, when 2-MG dissolves in bile acids, it plays the role of a surfactant by acting to increase the absorption of free fatty acids. Next, the free fatty acids and 2-MG biosynthesize bile acid compound micelles together with cholesterol and phospholipids, which are then incorporated into small intestine epithelial cells where the resynthesis of triacylglycerol takes place, after which this is ultimately released into the lymph in the form of chylomicrons.
However, persons that require arachidonic acid at the same time also have weak activity of pancreatic lipase (for example, pancreatic lipase activity is also known to decrease with aging), which is responsible for the first stage of oil/fat (triglyceride) absorption, and are unable to absorb adequate amounts of arachidonic acid from foods and oils and fats containing arachidonic acid (including arachidonic-acid containing oils and fats in the form of microbial fermented oils and fats).
Therefore, triglycerides in which medium-chain fatty acids, which are easily hydrolyzed by pancreatic lipase, are bound to the 1,3-positions of triglycerides and arachidonic acid is bound to the 2-position are the optimum oils and fats (triglycerides) for persons requiring arachidonic acid. Although Japanese Unexamined Patent Publication No. 8-214891 discloses a production method of an oil or fat containing triglyceride that contains polyunsaturated fatty acid wherein medium-chain fatty acids are bound to the 1,3-positions and a polyunsaturated fatty acid is bound to the 2-position, the only concrete description is that of a production method of triglyceride in which EPA or DHA is bound to the 2 position, while there is no specific disclosure whatsoever of a production method of triglyceride in which arachidonic acid is bound to the 2-position.
Japanese Unexamined Patent Publication No. 2000-270885 discloses a method for producing a structural lipid in which the number of carbon atoms of the fatty acids bound to the 1- and 3-positions of the target triglyceride is 12 or less, and 90% or more of the fatty acids bound to the 2-position are polyunsaturated fatty acids by allowing lipase to specifically act on the 1,3-positions of the triglyceride. Here, the oil or fat that allows the lipase to act is, for example, triglycerides in which 98% or more is EPA triglyceride, and this is synthesized by allowing non-position-specific lipase to act on glycerin and a polyunsaturated fatty acid or lower alcohol ester thereof while dehydrating. However, in the above method, although a highly pure polyunsaturated fatty acid or lower alcohol ester thereof is required instead of a mixture, as it is still difficult to obtain these inexpensively, it is not realistic to produce a target product by the aforementioned method.
On the other hand, a method is known for inexpensively producing an oil or fat (triglyceride) containing polyunsaturated fatty acid by fermentation. A production method of triglyceride in which caprylic acid is bound to the 1- and 3-positions, utilizing this microbial oil, was disclosed by Yuji Shimada (Journal of Fermentation and Bioengineering, 83, 321-327 (1997) “Fatty Acid Specificity of Rhizopus delemar Lipase in Acidolysis”) wherein a microbial oil containing 25% by weight of arachidonic acid that was available at the time as substrate was fermented by 1,3-position specific type lipase. However, as the position where the arachidonic acid binds to the triglyceride of this microbial oil is random, even if fatty acid at the 1- and 3-positions is nearly completely substituted by caprylic acid by the enzyme, the proportion of 1,3-capryolyl-2-arachidonoyl-glycerol (to be referred to as “8A8”) in the resulting oil does not exceed the proportion of arachidonic acid bound to the 2-position of the raw material oil even at the maximum level. In this case, the proportion of the arachidonic acid bound to the 2-position is at most 25% by weight and, in actuality, as there are also triglycerides present in which arachidonic acid is bound at multiple locations, the proportion of arachidonic acid bound to the 2-position is 25% by weight or less. According to the report by Shimada, et al. (Journal of Fermentation and Bioengineering, 83, 321-327 (1997)), although the resulting triglyceride was analyzed by high-performance liquid chromatography, as the retention times of 8A8,1,3-capryloyl-.gamma.-lino-lenoyl-glycerol (to be referred to as “8G8”) and 1,3-capryloyl-2-dihomo-.g-amma.-linolenoyl-glycerol (to be referred to as “8D8”) are the same, the proportion of 8A8 in the triglyceride was not accurately determined. However, as the total of 8A8, 8G8 and 8D8 was about 20 mol %, the resulting triglyceride was not satisfactory with respect to containing 25 mol % or more of 8A8.
In the case of using a microbial oil as a raw material oil in this manner, as the position where arachidonic acid binds to triglyceride is random, it is necessary to use, for the raw material, a triglyceride having a higher content of arachidonic acid in order to enhance the proportion of the target 8A8.
However, the reactivity of 1,3-position specific type lipase to fatty acid decreases the longer the length of the carbon chain and the greater the number of double bonds. In addition, the location of the double bonds, in terms of the carbon atoms at which double bonds are inserted when counting from the carboxyl group, is also an important element when discussing the reactivity of lipase. For example, although lipase exhibits a high level of reactivity with α-linolenic acid (9,12,15-octadecatrienoic acid), it exhibits extremely low reactivity with γ-linolenic acid (6,9,12-octadecadienoic acid), and although it exhibits high reactivity with DPA ω3 series (7,10,13,16,19-docosapentaenoic acid), it exhibits extremely low reactivity with DPA ω6 series (4,7,10,13,16-docosapentaenoic acid). Namely, lipase has the problem of exhibiting low reactivity with unsaturated fatty acids having 3 or more double bonds in the case of ω6 series polyunsaturated fatty acids having 18 or more carbon atoms, and unsaturated fatty acids having 2 or more double bonds in the case of ω9 series unsaturated fatty acids. Thus, in order to obtain an oil or fat containing a higher concentration of a target triglyceride in which medium-chain fatty acids are bound to the 1,3-positions and at least one polyunsaturated fatty acid, selected from the group consisting of ω6 series polyunsaturated fatty acid having 18 or more carbon atoms and 3 or more double bonds, and ω9 series polyunsaturated fatty acid having 18 or more carbon atoms and 2 or more double bonds, is bound to the 2-position, it is necessary to use an oil or fat containing a higher concentration of at least one type of polyunsaturated fatty acid, selected from the group consisting of ω6 series polyunsaturated fatty acid having 18 or more carbon atoms and 3 or more double bonds and ω9 series polyunsaturated fatty acid having 18 or more carbon atoms and 2 or more double bonds, for the raw material oil or fat. However, the higher the content of this oil or fat, the lower the reactivity and the poorer the reaction yield. This decrease in the reaction yield results in the formation of a large amount of unreacted triglyceride (raw material triglyceride and triglyceride in which only one of the fatty acids at the 1,3-positions have become a medium-chain fatty acid), and as a result, the proportion of the target triglyceride cannot be increased. Thus, there is a strong need for the development of a practical method for increasing the proportion of target triglyceride.
The ω6 series polyunsaturated fatty acid, dihomo-γ-linolenic acid, is expected to demonstrate precursor effects on type I prostaglandins, antithrombotic action, blood pressure lowering action, antidyskinetic action, anti-inflammatory action, delayed allergy inhibitory effects, skin protective action and anticancer action as its independent physiological actions. Thus, although there has similarly been a need for the development of triglyceride in which medium-chain fatty acids are bound to the 1,3-positions and dihomo-γ-linolenic acid is bound to the 2-position, the existence of oils and fats (triglycerides) having a high content of dihomo-γ-linolenic acid is not known, and there are no known findings whatsoever regarding the production of triglyceride for that purpose.
Fatty acids of ω9 series polyunsaturated fatty acids such as 5,8,11-eicosatrienoic acid (20:3 ω9 series, to be referred to as Mead acid) and 8,11-eicosadienoic acid (20:2 ω9 series) are known to be present as one of the constituent fatty acids in animal tissue deficient in essential fatty acids. However, since they are only present in minute amounts, their isolation and purification has been extremely difficult. These polyunsaturated fatty acids are able to become precursors of the leukotriene 3 group in the body, and their physiological activity is the target of considerable expectation and reported examples of which include anti-inflammatory, antiallergic and anti-rheumatic action (Japanese Unexamined Patent Publication No. 7-41421). Thus, although there is similarly a need for the development of triglycerides in which medium-chain fatty acids are bound to the 1,3-positions and ω9 series polyunsaturated fatty acid is bound to the 2-position, the existence of oils and fats (triglycerides) having a high content of ω9 series polyunsaturated fatty acid is unknown, and there are no known findings whatsoever relating to the production of a triglyceride for that purpose.